US 2007/0161269 corresponding to JP-A-2006-303327 discloses an electronic circuit device configured as an electronic key transceiver The electronic circuit device includes a circuit board having first and second sides opposite each other. Electronic elements are mounted on only the first side of the circuit board. The circuit board is encapsulated in a casing in such a manner that the second side of the circuit board is exposed to an outer surface of the casing. Thus, the second side of the circuit board defines part of the outer surface of the casing.
A method of making the electronic circuit device includes a placing process and an encapsulating process subsequent to the placing process. In the placing process, the circuit board is placed in a cavity of a mold (i.e., die) in such a manner that the second side of the circuit board can be held in close contact with an inner surface of the cavity. In the encapsulating process, a liquid resin material is injected under pressure into the cavity of the mold and then cured (i.e., solidified).
Since the circuit board is held in close contact with the inner surface of the cavity, the circuit board can be prevented from being greatly deformed by pressure and heat, which are applied to the circuit board in the encapsulating process.
In the method described above, after the resin material is injected to the cavity, air may be trapped in a gap between the electronic element and the circuit board. The air trapped in the gap may expand due to the pressure and heat and cause a bulge (bump or the like) on the second side of the circuit board. Further, the second side of the circuit board may jut out due to a difference in coefficients of linear (thermal) expansion between the circuit board and the casing. Since the second side of the circuit board is exposed to the outer surface of the casing, the bulge and the jut mar the appearance of the electronic key transceiver.
Such a deformation on an exposed side of the circuit board can be eliminated by a sanding technique or the like. However, generally, the deformation on the exposed side of the circuit board is very small, has a complex shape, and differs from product to product. Therefore, it is difficult to automate an eliminating process to eliminate the deformation, and the eliminating process may be performed manually. As a result, it takes a lot of man hours to eliminate the deformation. Further, the manual eliminating process may result in a decrease in accuracy (i.e., flatness) of the second side of the circuit board.